Dark Foundry

The essence of effective Dark Eldar strategy is the manufacture of fast-attack vehicles capable of easily traversing terrain and engaging enemy forces in raids. The Dark Foundry supplies the commander with a vast array of vehicles capable of meeting this task. In addition, the centuries-old engineers conduct research to improve the quality and durability of their weapons.

Tactics

Unlike ALL other vehicle producing buildings, The Dark Foundry has no building requirements and can produce the versatile Hellions immediately making the Dark Foundry a viable alternative to the Hall of Blood as a first building. Reaver Jetbikes in early game are very effective at harassing. The only drawback is that it doesn't produce anything to counter enemy commanders the way an Archon would.

In the long run you'll want to build a Dark Foundry anyway but if you favor faster, more aggressive harassment in the early game, build the Dark Foundry first.